The present invention relates to a resin molded type semiconductor device, which is assembled by making use of a lead frame with a heat radiation plate.
These days, significant advances are being made in higher integration and higher performance in semiconductor devices; and, in association therewith, the amount of heat generated in a semiconductor chip has tended to increase. In order to dissipate the heat generated by a semiconductor chip, copper or a copper alloy (small amount of such elements as Ag, Sn, Fe, Cr, Zn, Ni, Mg, P and Si is added for enhancing such characteristics as mechanical strength) having excellent heat conductivity is currently being used as the material of a lead frame in place of the conventionally used 42 alloy (42%Nixe2x80x94Fe alloy).
Further, since the amount of heat generated in a microcomputer is particularly high, an efficient heat dissipation therefor is required; and, for this purpose, there is a package using a lead frame having a heat radiation plate, in which the heat radiation plate is connected via an adhesive layer to the lead frame, known as a HQFP (Quad Flat Package with Heat Sink).
FIG. 18 shows in a top plan view an example of a conventional HQFP; FIG. 19 shows a structural example of the conventional HQFP; and FIG. 20 shows in cross-section an inner plane view of the conventional HQFP. The conventional HQFP is generally assembled in the following manner.
At first, as seen in FIG. 19, a heat radiation plate 3, on which an adhesive layer 2, such as polyimide resin, is formed in advance, such as by coating, is secured to an inner lead portion 1a of a lead frame 1 after being joined, heat pressed and cured. Subsequently, a semiconductor chip 4 is adhered on the heat radiation plate 3 or on a die pad of the lead frame by an adhesive member 5, such as Ag paste.
Then, electrodes on the semiconductor chip and top ends of the inner lead are connected by metallic fine wires 6 made of a conductive material, such as gold. In this instance, at least on a portion where the metallic fine wire of the inner lead is connected, a plating 7 is frequently applied in advance to form a metallic fine wire connection using Ag, so as to obtain good connectability. Thereafter, the semiconductor chip 4, the metallic fine wire 6, the inner lead 1a and a part of the (or the entire) radiation plate 3 are covered by a molding resin 8, such as epoxy resin; and, finally, a portion of an outer lead 1b of the lead frame 1 is plated and then bent to form the outer lead 1b, whereby the device is completed after a marking is applied thereto.
Before shipping the semiconductor device to the market, a variety of reliability tests are performed thereon. Among these tests, there is a humidity acceleration test called a PCT (Pressure Cooker Test); and, when HQFP of conventional structure is subjected to such a PCT, problems tend to arise in that a deterioration phenomena, such as leakage and shorting, are caused about 200 hours after the start of the test. As a result of an analysis of the deterioration phenomena due to PCT by the inventors, it was found that the deteriorations are caused for reasons that will be explained below.
FIGS. 21 and 22 are cross-sectional views of a HQFP having a conventional structure, as seen along line Axe2x80x94A shown in FIG. 19, and the above-mentioned problems will be explained in detail with reference to FIGS. 21 and 22. FIG. 21 shows the cross-section before the device is subjected to a PCT, and FIG. 22 shows the cross-section after the PCT, in which the lead frame 1 is joined to the heat radiation plate 3 via the adhesive layer 2 and these elements are covered by the molding resin 8.
The PCT is performed at a high temperature of 121xc2x0 C. Further, there are differences in the coefficients of thermal expansion of the respective materials. For example, the coefficient of thermal expansion of the molding resin 8 is 10-30 ppm/xc2x0 C., the coefficient of the thermal expansion of copper or copper alloy of the lead frame 1 and heat radiation plate 3 is about 17 ppm/xc2x0 C. and the coefficient of thermal expansion of the adhesive layer 3 is 30-40 ppm/xc2x0 C. Thus, a first problem arises in that, after the PCT, as shown in FIG. 22, a peeling off portion 9 is produced at respective boundaries between the lead frame 1 and the adhesive layer 2 and between the molding resin 8 and the adhesive layer 2.
Further, when the peeling off portion 9 is produced at respective boundaries between the lead frame 1 and the adhesive layer 2 and between the molding resin 8 and the adhesive layer 2, since the PCT is performed under a severe condition of 121xc2x0 C./100% RH/2 atm, moisture penetrates into the semiconductor device through the boundary between the lead frame 1 and the molding resin 8 or through the molding resin 8 itself, and the moisture collects inside the space created by the peeling off portion 9.
In regard to the moisture collected in the peeling off portion 9, components such as the molding resin 8, the adhesive layer 2 and the paste material 5 have been extracted, and it has been found that the collected moisture exhibits an acidic character. The extracted components are organic acid and chlorine ions contained in the molding material, or a component which changes the extracted liquid into acid. By this acidic solution, the copper or the copper alloy, which is the material of the lead frame 1, is eluted and ionized and is redeposited as deposited copper 10, thereby, a phenomenon (ion migration) arises that represents a second problem which results in a shorting between the leads.
Further, at the top end portion of the inner lead 1a where the plating 7, such as Ag, for connecting the metallic fine wire 6, the plating metal 7 and the copper or the copper alloy, which is the material of the lead frame 1, are exposed to the moisture at the same time, a galvanic cell is formed by the dissimilar metal junction, and so the above phenomenon is further accelerated.
FIGS. 23 and 24 show the area around the end portion (B portion) of the heat radiation plate 3 in the HQFP having a conventional structure, as shown in FIG. 19. FIG. 23 shows the B portion before the PCT, and FIG. 24 shows the B portion after the PCT, in which the lead frame 1 is joined to the heat radiation plate 3 via the adhesive layer 2 and these elements are molded by the molding resin 8.
Even at a heat radiation plate end portion 3a, the peeling off portion 9 is likely caused after the PCT, as shown in FIG. 24, and moisture accumulates at the peeling off portion 9. Due to the presence of this accumulated acidic water, the copper or the copper alloy, which is the material of the heat radiation plate 3, is eluted and ionized, and redeposited as the deposited copper 10, thereby, a problem of causing shorting between the lead frame 1 and the heat radiation plate 3 arises.
Further, JP-A-10-163410 (1998) proposes a technique for preventing ion migration preventing in a taping lead frame, in which a protective film is formed at a portion of a lead contacting an adhesive. However, an object of the technique disclosed in JP-A-10-163410 (1998) is to prevent copper diffusion movement through the adhesive in the taping lead frame caused by an electric field formed by voltage application, and so the structure of the concerned lead frame with the heat radiation plate and the semiconductor device, and the ion migration phenomenon are different.
Further, the ion migration of the copper diffusive movement through the adhesive is resolved by changing the adhesive material from a phenol resin series to a maleimide resin series and polyimide resin series, for example.
Further, JP-A-8-20498 (1996) proposes a lead frame with a heat radiation plate in which, in order to prevent electrical shorting between the lead frame and the heat radiation plate end portion due to the presence of a heat radiation plate punching burr, an insulative coating is provided for the lead frame at the face joining the adhesive layer, and the insulative coating is formed so as to protrude from the heat radiation plate end portion. However, with this measure, when peeling off is caused, the migration between leads and between a lead and a heat radiation plate can not be prevented. In particular, JP-A-8-204098 (1996) nowhere discloses migration between the leads. Accordingly, for example, when dealing with a narrow pitch type semiconductor device in which the interlead pitch is narrow, the technical countermeasure disclosed in JP-A-8-204098 (1996) is insufficient when applied to migration.
An object of the present invention is to solve the above-described first and second problems and to provide a semiconductor device which limits generation of peeling and cracks and prevents problems, such as leakage and shorting, due to ion migration, even when the peeling and cracks appear to some extent.
The above and other objects and novel features of the present invention will become apparent from the description provided in the present specification and from the attached drawings.
Among aspects and features of the invention disclosed in the present application, an outline of typical ones will be briefly explained hereinbelow.
Namely, a semiconductor device according to one aspect of the present invention comprises a plurality of inner leads, which are made of copper or a copper alloy and extend around a semiconductor chip; a heat radiation plate which is joined to one end of the plurality of inner leads via an insulative adhesive layer and on which the semiconductor chip is mounted via the adhesive layer or an adhesive different from the adhesive layer; a plurality of metallic fine wires which connect between the semiconductor chip and the plurality of inner leads, respectively; and, a molding resin which molds the semiconductor chip, the plurality of metallic fine wires and the heat radiation plate, wherein a portion of the inner leads which joins with the adhesive layer is covered by a metal having a higher reference electrode potential than that of copper.
Further, in a semiconductor device according to another aspect of the present invention, wherein a lead frame and a heat radiation plate, which are made of copper or copper alloy, are joined by an adhesive layer formed on a surface of the heat radiation plate, the device is assembled by making use of the lead frame with the heat radiation plate. In addition, at least a part of the inner leads are provided with a metallic fine wire connection plating, and at least the entire portion where the lead frame joins the adhesive layer is covered by at least one metal or alloy different from the material of the metallic fine wire connecting plating. For example, such metal or alloy is selected from the group consisting of gold, platinum, iridium, rhodium, palladium, ruthenium, indium, tin, molybdenum, tungsten, gallium, zinc, chromium, niobium, tantalum and titanium.